Re: CAP_SYSLOG, 2.6.38 and user space

[Gergely Nagy <algernon@balabit.hu>]

On Wed, 2011-02-09 at 13:34 -0800, david@lang.hm wrote: 
> On Wed, 9 Feb 2011, Gergely Nagy wrote:
> 
> > On Wed, 2011-02-09 at 21:23 +0000, Serge E. Hallyn wrote:
> >> So if that's how we're leaning, then the following patch is much more
> >> concise.  I'll send this to Linus and any appropriate -stable tomorrow
> >>  if noone objects.
> >>
> >> From 5166e114d6a7c508addbadd763322089eb0b02f5 Mon Sep 17 00:00:00 2001
> >> From: Serge Hallyn <serge@hallyn.com>
> >> Date: Thu, 3 Feb 2011 09:26:15 -0600
> >> Subject: [PATCH 1/1] cap_syslog: don't refuse cap_sys_admin for now (v2)
> >>
> >> It'd be nice to do that later, but it's not strictly necessary,
> >> and it'll be hard to do without breaking somebody's userspace.
> >>
> >> Signed-off-by: Serge Hallyn <serge@hallyn.com>
> >> ---
> >>  kernel/printk.c |   14 ++++----------
> >>  1 files changed, 4 insertions(+), 10 deletions(-)
> >
> > Personally, I'd prefer the sysctl idea in the long run, because
> > userspace can easily and automatically adapt to the running kernel then.
> > Ie, this patch is fine for 2.6.38, but later on, a sysctl could be
> > introduced, that when set (but defaulting to unset, as to not break
> > userspace), would make CAP_SYS_ADMIN return -EPERM. That way, syslogds
> > could look at the setting, and act accordingly. This would mean that old
> > userspace wouldn't break, and upgraded userspace could work on both old
> > and new kernels, depending on the setting. Distros or admins could then
> > enable the sysctl once they made sure that all neccessary applications
> > have been upgraded.
> 
> what is your justification for ever having CAP_SYS_ADMIN return -EPERM? 
> what's the value in blocking this.

Nothing. Come to think of it, the main use of the sysctl would be to
detect CAP_SYSLOG support, so that applications can drop CAP_SYS_ADMIN
and use CAP_SYSLOG only (which, imo, is a good idea - the less
capabilities, the better, and CAP_SYS_ADMIN is quite broad when one only
wants CAP_SYSLOG).

If there's a better way to allow userspace to easily detect CAP_SYSLOG,
I'm all for that.

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